This invention relates to a light source device, and more particularly a light source that is used as a light source for a projector or a light source for a fiber lighting system.
In general, in the case of a light source device used as a light source for a projector or a light source for a fiber lighting system, it is necessary to achieve that light radiated from a light source lamp is collected efficiently and radiated against an area to be irradiated. Normally, such a light source device as described above is constituted by a short arc discharge lamp and a concave reflector for collecting light radiated from the discharge lamp.
In recent years, it has been required to provide a small-size light source device in which the rate of utilization of light radiated from the short arc discharge lamp is high.
FIG. 7 is an illustrative view showing one example of a configuration of this kind of light source device in the prior art. This light source device 50 is constituted such that a short arc discharge lamp 51 is assembled in a concave reflector 58.
A discharge container of the short arc discharge lamp 51 is constituted by a light emitting bulb 52 and sealing parts 53 extending at both ends of the light emitting bulb 52, wherein a cathode 54 and an anode 55 are arranged opposite to each other within the light-emitting bulb 52.
The light-emitting bulb 51 of the discharge container is formed to have a rugby-ball type spinning barrel shape, for example, having a large inner surface area in view of its purpose to reduce the load on the bulb wall and prevent the phenomenon of devitrification of the light-emitting bulb 51.
In turn, the concave reflector 58, for example, is a mirror having an elliptical surface with an optical axis L, the discharge lamp 51 is arranged such that it coincides with the optical axis L in its arc discharge direction, and a bright spot position A of the arc formed between the cathode 54 and the anode 55 (hereinafter called xe2x80x9ca center of the arcxe2x80x9d) is coincident with a first focal point of the concave reflector 58.
In the case of the light source device 50, light radiated toward a rear region positioned in a rear part (the left side in the figure) of a virtual straight line N connecting any point M on the front outer edge of the concave reflector 58 with the center A of arc is collected by the concave reflector 58 and radiated against the area to be irradiated.
However, light radiated toward the front region positioned in front of the virtual straight line N, for example, almost all light I3 of the radiated light I11 radiated from the center A of the arc in one possible forward direction P, passes through the bulb wall of the light emitting bulb 52, and the light cannot be collected by the concave reflector 58, with the result that the light cannot be effectively utilized.
In turn, the part I12 of the light of the radiated light I11 is reflected by an inner surface 52a of the bulb wall of the light emitting bulb 52, and the part of light I22 of light I21 which penetrated an inner surface and passed through the inner surface 52a of the bulb wall is reflected by the outer surface 52b of the bulb wall. In this case, the rate of light reflected by both surfaces reaches 8% of the incident light.
However, since the directions of the inner surface reflecting light I12 and the outer surface reflecting light I22 are not appropriate, or the light is shut off by the electrodes, or a part of it is absorbed by the electrodes, both types of light cannot be utilized effectively.
In order to increase the rate of utilization of light radiated from the short arc discharge lamp, the specification of U.S. Pat. No. 4,305,099, for example, provides a light source device having a constitution in which a ring-like auxiliary concave reflector is arranged at a forward position of the concave reflector in such a way that the first focal point is coincident with a first focal point of the concave reflector.
However, actually, it is quite hard to arrange the auxiliary concave reflector at a proper position with high accuracy so that at last there occurs the problem that the characteristic of a spot light source of the short arc type discharge lamp is lost.
As described above, in the case of the light source device utilizing a prior art short arc discharge lamp, actually, the light reflected at the light-emitting bulb cannot be utilized effectively and a high rate of utilization of light cannot be realized.
An object resolved by the present invention is to provide a small-size light source device having a simple construction and showing a high rate of utilization of light radiated from the short arc discharge lamp.
In order to solve the problem described above, the light source device of the present invention is comprised of a short arc discharge lamp in which a pair of electrodes are arranged opposite within a light emitting bulb of a discharge container, and a concave reflector is arranged in such a way that the arc direction of the discharge lamp is coincident with the optical axis, wherein a distance between the electrodes in the short arc discharge lamp is 4.0 mm or less, the light emitting bulb of the short arc discharge lamp, at least in a specific portion of its inner surface and its outer surface, has a spherical surface surrounding a center (A) of the arc, wherein the specific portion is in a front region in front of a critical straight line (D) and is defined by a solid angle of at least 3 sr or more, with the center (A) of the arc being an apex and the optical axis (L) of the concave reflector being a central axis, and wherein the critical straight line (D) is a straight line connecting the center (A) of the arc with a point (Y) corresponding to a cross point where an extended line of a virtual straight line (N) connecting any point (M) on the front outer edge of the concave reflector with the center (A) of the arc intersects the outer surface of the bulb wall of the said light emitting bulb.
Further, in the case of the aforesaid light source device, it is preferable that the outer surface at least in the specific portion of the light emitting bulb of the short arc discharge lamp is provided with a reflective film.
Further, in the case of the aforesaid light source device, the short arc discharge lamp can be made such that a cathode and an anode are arranged opposite to each other within a light emitting bulb in a discharge container, the cathode is arranged within the concave reflector while being positioned at a forward position, the light emitting bulb has a form elongated in a direction of the optical axis (L) within a rearward region where it is positioned at a more rear part than the critical straight line (D).
In accordance with the aforesaid configuration, a part of the light radiated toward the front region can be utilized advantageously, resulting in that a high rate of utilization of light can be realized. That is, the inner surface and the outer surface of at least the specific portion in the front region of the light emitting bulb of the short arc type discharge lamp are spherical surfaces, the center of the spherical surface is coincident with the center of the arc, whereby the light reflected by these surfaces is returned back to the arc region, resulting in that the reflected light can be utilized effectively by the concave reflector.
In the case that a reflective film is formed on the outer surface in at least the specific portion in the front region of the light emitting bulb of the short arc type discharge lamp, the light which must pass through the outer surface of the bulb wall of the light emitting bulb is also reflected and returned back to the arc region, resulting in that almost all of the light radiated toward the front region can be utilized effectively by the concave reflector and at the same time the constitution of the light source device can be made simple and small in size.